Known in the art is an internal combustion engine having arranged in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NOx when the oxygen concentration in the inflowing exhaust gas falls. In this internal combustion engine, the NOx produced when burning fuel under a lean air-fuel ratio is stored in the NOx storing catalyst.
However, when using such an NOx storing catalyst, it is necessary to make the NOx storing catalyst release the NOx before the NOx storing capability of the NOx storing catalyst becomes saturated. In this case, if making the air-fuel ratio of the exhaust gas flowing into the NOx storing catalyst rich, it is possible to make the NOx storing catalyst release the NOx and to reduce the released NOx. Therefore, in conventional internal combustion engines, the NOx storing catalyst is made to release NOx by making the air-fuel ratio in the combustion chamber rich or by feeding fuel into the engine exhaust passage upstream of the NOx storing catalyst to make the air-fuel ratio of the exhaust gas flowing into the NOx storing catalyst rich.
However, to make an NOx storing catalyst release NOx well, sufficiently gasified rich air-fuel ratio exhaust gas has to be made to flow into the NOx storing catalyst. In this case, if making the air-fuel ratio in the combustion chamber rich, the sufficiently gasified rich air-fuel ratio exhaust gas flows into the NOx storing catalyst, so it is possible to make the NOx storing catalyst release the NOx well. However, if making the air-fuel mixture in the combustion chamber rich, there is the problem that a large amount of soot is produced. Further, if injecting additional fuel into the expansion stroke or exhaust stroke so as to make the air-fuel ratio of the exhaust gas exhausted from the combustion chamber rich, the injected fuel sticks to the inside walls of the cylinder bore, i.e., bore flushing occurs.
As opposed to this, when injecting fuel into the engine exhaust passage upstream of an NOx storing catalyst, the problems of soot being produced or bore flushing occurring as explained above no longer arise. However, when injecting fuel into the engine exhaust passage upstream of the NOx storing catalyst, there is the problem that the injected fuel is not sufficiently gasified and therefore the NOx storing catalyst cannot be made to release NOx well.
On the other hand, known in the art is an internal combustion engine arranging a hydrocarbon, that is, HC adsorbing catalyst for adsorbing HC contained in exhaust gas in the engine exhaust passage upstream of the NOx storing catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 2003-97255). In this internal combustion engine, the HC produced when burning fuel under a lean air-fuel ratio is adsorbed by the HC adsorbing catalyst and the NOx produced at that time is stored in the NOx storing catalyst.
However, in this internal combustion engine, when the temperature of the HC adsorbing catalyst becomes near the activation temperature, that is, near 200° C., the oxidation reaction of the adsorbed HC becomes active and as a result the oxygen in the exhaust gas is rapidly consumed, so the oxygen concentration in the exhaust gas rapidly falls. Therefore, at this time, if additionally supplying a small amount of fuel, it is possible to make the air-fuel ratio of the exhaust gas rich. Therefore, in this internal combustion engine, it is detected whether a sufficient amount of oxygen has been consumed at the HC adsorbing catalyst, and the air-fuel ratio of the exhaust gas is made rich when a sufficient amount of oxygen is being consumed in the HC adsorbing catalyst so as to make the NOx storing catalyst release NOx.
However, in this internal combustion engine, the air-fuel ratio in the combustion chamber is made rich. Fuel is not injected into the engine exhaust passage. Therefore, the above problem arises. Further, in this internal combustion engine, the period when the temperature of the HC adsorbing catalyst becomes near the activation temperature, that is, the period when a sufficient amount of oxygen is consumed in the HC adsorbing catalyst, is limited, so the temperature of the HC adsorbing catalyst will not become the activation temperature in the period required as seen from the action of the NOx storing catalyst releasing the NOx and consequently there is the problem that the NOx storing catalyst cannot release NOx when the NOx storing catalyst has to release the NOx.